Implantable devices are provided for stabilizing adjacent vertebrae and the lumbosacral region of a patient. The devices can comprise an interspinous flexible spacer body having a substantially U-shape comprising a superior section, inferior section, and a midsection extending therebetween. The superior and/or inferior sections can include a pair of lateral walls configured to engage a spinous process of a vertebra. Fixation caps can be provided for securing a spinous process of a vertebra to the flexible spacer body. To secure the flexible spacer body between the lumbar vertebra and an adjacent vertebra, an anchor assembly is provided. Also provided are methods of using the implantable devices to stabilize a patient's spine.